The present invention relates to a single common rotating polygon mirror in a raster output scanning (ROS) system for multiple photoreceptors and, more particularly, to a single rotating polygon mirror with adjacent facets having different tilt angles to reflect and split the scanning beam to multiple photoreceptors.
Printing systems utilizing lasers to reproduce information are well known in the art. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoreceptor to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. This laser beam is modulated in conformance with image information. The modulated beam is transmitted through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive medium. The rotation of the polygon causes the spot to scan across the photoreceptor in a fast scan (i.e. line scan) direction. Meanwhile, the photoreceptor is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the photoreceptor recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image information serial data stream so that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photoreceptor to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. Laser printers may operate in either a single pass or a multiple pass system.
In a single pass, process color system, three ROS systems are positioned adjacent to a photoreceptor surface and selectively energized to create successive image exposures, one for each of the three basic colors. A fourth ROS system may be added if black images are to be created as well. In a multiple pass system, each image area on the photosensitive medium must make at least three passes relative to the scan line formed by the modulated laser beam generated by a single ROS system.
Problems with these color printing systems include the high cost related to the use of multiple ROSs, the high cost of producing nearly identical multiple ROSs and associated optics, and the difficulty of aligning or registering the system colors.
Traditionally, a single beam ROS has a single light source which emits a single modulated light beam which is reflected from the facets of the rotating polygon mirror to scan a single line on a single photoreceptor. Three or four ROS systems are used to provide color printing.
A multiple beam ROS has either a single light source which emits two or more different modulated light beams or multiple light sources which emit multiple different modulated light beams. These multiple beams are collimated by the same single collimated lens and then focused by the same single cylindrical lens onto the facets of a single rotating polygon mirror. After reflecting from the facet, the multiple beams pass through f-theta scan lenses and motion compensating optical elements to scan multiple lines on a single photoreceptor.
One successful way to increase the photoreceptor speed is to employ multiple or "multispot" diode lasers. A multispot diode laser is a single device that has a plurality of closely spaced semiconductor lasers. The use of multispot diode lasers enables two or more independently addressable laser beams to be modulated together, thereby increasing the number of scan lines that are discharged across the photoreceptor during a single sweep.
Typically, the facets of the polygon mirror are uniform in shape and uniform in tilt angle relative to the axis of rotation. Any minute differences in size and angle from facet to facet are treated as "wobble" or "bow" errors in the ROS to be corrected by lens and mirrors before the light beam is scanned across the photoreceptor.
A bar code reader patent to Takenaka (U.S. Pat. No. 5,223,700) proposed the facets of a polygon mirror have different lengths along the circumference of the mirror. A wider facet gives a longer scan line while a shorter facet gives a shorter scan line.
A laser beam scanner patent to Conemac (U.S. Pat. No. 5,646,766) suggests tilting the facets of a polygon mirror in the y-direction along the fast scan (i.e. line scan) direction to space the scanning light beam spots closer together on the scan line.
However, both the Takenaka and Conemac patents have the facets of the polygon mirror adjust the spot distribution on a single scan line on a single photoreceptor.
It is an object of this invention to provide a single rotating polygon mirror with adjacent facets having different tilt angles to reflect and split the scanning beam to multiple photoreceptors in a raster output scanning (ROS) system.